Power system with shared clamp reset

ABSTRACT

An example power supply includes a first power converter, a second power converter, and a shared clamp reset circuit. The first power converter is adapted to convert an input to a first output and includes a first transformer having a first primary winding. The second power converter is also adapted to convert the input to a second output and includes a second transformer having a second primary winding. The second primary winding of the second transformer is not the first primary winding of the first transformer. The shared clamp reset circuit is coupled to the first primary winding of the first transformer and is coupled to the second primary winding of the second transformer to manage leakage inductance energy within the first transformer and within the second transformer.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/437,481, filed on May 7, 2009 and now pending. U.S. patentapplication Ser. No. 12/437,481 is hereby incorporated by reference.

The application is related to U.S. patent application Ser. No.12/234,519, filed on Sep. 19, 2008 and now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power supplies, and morespecifically to power supplies typically utilized in electricalequipment such as a printer, or the like that uses a standby power and amain power supply.

2. Discussion of the Related Art

Particular types of power supplies may convert the alternating current(AC) line delivered from a wall socket to the direct current (DC) powerusable with today's electronics. In some circumstances, power suppliesgenerally utilize two separate power supplies within the power supplyitself.

In one example, a personal computer (PC) may use a power supply whichincludes a main power supply and a standby power supply. Both the mainand standby power supplies relay power to other components comprising aPC. The PC main supplies the main power, which supplies power to themotherboard and processor of a PC. In addition, the PC main is designedto provide a large amount of output power (which is typically between200-400 watts) to supply the motherboard and additional components, suchas disk drives and video cards. On the other hand, the standby powersupply provides a lower amount of output power (which is typically lessthan 5 or 10 watts).

Unlike the main power supply, which powers down when a user hasindicated shut down of the PC (or other electronic device), the standbypower supply normally does not shut down and (after being powered up)continues to convert an input voltage into an appropriate outputvoltage. Thus, the main power supply operates on an as-needed basis,while the standby power supply continues to operate whenever an inputvoltage is present (i.e. the power supply is plugged into a wallsocket). The standby power supply then provides power to componentswhich continue to run once the PC (or other electronic device) has beenpowered down in response to a user indication. For example, the standbypower supply provides power to the power button of a computer so a usercan use the power button to manually start the PC (which provides anindication to power up or power down the main power converter). Inanother application, the main and standby power supply may be utilizedwith a printer. The standby power supply may provide power to thenetwork connection of a printer such that the printer may monitor forany incoming print job requests while in a standby (or idle) mode andbecome active when a print job request is received.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of severalembodiments of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings.

FIG. 1 illustrates a system block diagram of a power supply system inaccordance with one embodiment of the present invention.

FIG. 2 illustrates a schematic diagram of a main power supply utilizedwith the power supply system of FIG. 1 in accordance with one embodimentof the present invention.

FIG. 3A illustrates another schematic diagram of a main power supplyutilized with the power supply system of FIG. 1 in accordance withanother embodiment of the present invention.

FIG. 3B illustrates another schematic diagram of a main power supplyutilized with the power supply system of FIG. 1 in accordance withanother embodiment of the present invention.

FIG. 4 illustrates a schematic diagram of a standby power supplyutilized with the power supply system of FIG. 1 in accordance with oneembodiment of the present invention.

FIG. 5A illustrates a schematic diagram of a clamp reset circuitutilized with the power supply system of FIG. 1 in accordance with oneembodiment of the present invention.

FIG. 5B illustrates another schematic diagram of a clamp reset circuitutilized with the power supply system of FIG. 1 in accordance with oneembodiment of the present invention.

FIG. 5C illustrates a further schematic diagram of a clamp reset circuitutilized with the power supply system of FIG. 1 in accordance with oneembodiment of the present invention.

FIG. 6 illustrates another schematic diagram of a main power supplyutilized with the power supply system of FIG. 1 in accordance withanother embodiment of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

The following description is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles ofexemplary embodiments. The scope of the invention should be determinedwith reference to the claims.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

As mentioned above, a power supply may utilize a main power supply and astandby power supply in operation. The main power supply is typicallydesigned with a two-switch forward converter. The two-switch forwardconverter is a low cost configuration that is suited for power suppliesfor personal computers and similar applications. The two-switch forwardconverter also has the advantages of producing high output current athigh efficiencies. Alternatively, the main power supply may also bedesigned with a single-switch forward converter. For both of theseforward converter topologies, an additional reset circuit may beutilized to manage the magnetizing energy of the transformer used ineither the two-switch or single-switch forward converter topology.

On the other hand, the standby power converter is often designed with aflyback converter. The flyback converter typically provides low outputcurrents at low component cost. In addition, the flyback converter mayutilize an additional clamp circuit which prevents excess voltage fromdamaging components within the flyback converter. However, it should beappreciated that the standby power converter may be designed with atwo-switch or single-switch forward converter topology or the two-switchflyback converter topology. Typically, each converter topology used inthe power supply utilizes its own reset circuit or clamp circuit.

In accordance with the teachings of the invention, the main power supplyand the standby power supply may both utilize a single clamp resetcircuit. The clamp reset circuit provides substantially similaradvantages as the reset circuit typically utilized with the main powersupply and further provides similar advantages as the clamp circuittypically utilized with the standby power supply. The components of theclamp reset circuit may be found within the main power supply and thestandby power supply may electrically couple to the same clamp resetcircuit. Alternatively, the clamp reset circuit may be found withinstandby power supply and the main power supply may electrically coupleto the same clamp reset circuit. In other examples, the clamp resetcircuit is a separate entity to which both the main power supply and thestandby power supply electrically couple. By allowing the main powersupply and the standby power supply to share the same clamp resetcircuit, the teachings of the present invention may utilize fewercomponents and may provide a lower cost system than standard main andstandby power supply designs used in electrical equipment, such as PCpower supplies, printers, or the like.

Referring first to FIG. 1, a system block diagram of a power supplysystem 100 in accordance with one embodiment of the present invention isillustrated including a power supply 102, a main power supply 104, aclamp reset circuit 106, an input (V_(IN)) 108, a main output (V_(MO))110, a standby power supply 112, a standby output (V_(SO)) 114, a clampstandby connection 116, and an input return 118.

The power supply 102 includes a main power supply 104. The main powersupply 104 further includes a clamp reset circuit 106, which is coupledto other components within the main power supply 104, and main output(V_(MO)) 110. The main power supply 104 is coupled to the input (V_(IN))108, the standby power supply 112, and the input return 118 such thatthe voltage across the main power supply 104 is substantially equivalentto the input (V_(IN)) 108. In addition, the standby power supply 112 isalso coupled to the input (V_(IN)) 108 and input return 118 such thatthe voltage across the standby power supply 112 is substantiallyequivalent to the input (V_(IN)) 108. The standby power supply 112 isalso coupled to the standby output (V_(SO)) 114. Through the clampstandby connection 116, the clamp reset circuit 106 is coupled tocomponents comprising the standby power supply 112. The input (V_(IN))108, main power supply 104 and standby power supply 112 are coupled toinput return 118. The input return 118 provides the point of lowestpotential, or in other words the point of lowest voltage with respect tothe input (V_(IN)) 108, for the power supply system 100.

Within the power supply 102, the main power supply 104 comprisescircuitry to convert the received input (V_(IN)) 108 into an appropriateoutput (and in some embodiments, one or more output levels). In oneembodiment, the input (V_(IN)) 108 may be a rectified AC voltage. In oneexample, the main power supply 104 utilizes a forward converter topologyto convert the incoming power to the desired output level (furtherillustrated with respects to FIGS. 2 and 3). The main power supply 104provides the output to one or more devices external from the powersupply 102. However, the main power supply 104 can also provideappropriate output levels to devices which are internal to the powersupply 102. It should be appreciated that the main power supply 104 maycomprise one or more main outputs (V_(MO)) 110 which provide power toone or more devices. It should also be appreciated that the main output(V_(MO)) 110 may also output voltages, currents, or a combination ofboth. In one embodiment, the main power supply 104 provides a variety ofdifferent output levels along each main output (V_(MO)) 110. Forexample, the main power supply 104 may provide output voltages such as3.3 Volts (V), 5 V, or +/−12V. In addition, the main power supplyfurther comprises circuitry of the clamp reset circuit 106. The use ofthe clamp reset circuit 106 allows the main power supply 104 to managethe magnetizing energy and the leakage inductance energy within thetransformer of the main power supply 104. In other words, the clampreset circuit 106 allows the main power supply 104 to operate at alarger range of input voltages.

The standby power supply 112 also comprises circuitry to convert theinput (V_(IN)) 108, which comprises a rectified AC voltage in someembodiments, into an appropriate output level. In some examples, thestandby power supply 112 utilizes a flyback converter topology toconvert the incoming power to the desired output level (and in someembodiments, one or more output levels). The flyback converter topologyis further illustrated with respect to FIG. 4. However, in otherembodiments the standby power supply 112 may also utilize a two-switchor single-switch forward converter topology or the two-switch flybackconverter topology. Output from the standby power supply 112 is providedto one or more devices external to the power supply 102. The standbypower supply 112 can provide appropriate output levels to devices whichare internal to the power supply 102. However, it should be appreciatedthat the standby power supply 112 may comprise one or more standbyoutputs (V_(SO)) 114. It should also be appreciated that the standbyoutput (V_(SO)) 114 may also output voltages, currents, or a combinationof both. In addition, the standby power supply 112 may also be adaptedto provide a variety of different output levels along each standbyoutput (V_(SO)) 114. For example, the standby power supply 112 mayprovide a variety of different voltage levels along each standby output(V_(SO)) 114 such as 3.3 Volts (V), 5 V, or 12V. The clamp standbyconnection 116 electrically couples the standby power supply 112 to theclamp reset circuit 106. Use of the clamp reset circuit 106 with thestandby power supply 112 allows the standby power supply 112 to managethe leakage inductance energy within the transformer or coupled inductorof the standby power supply 112 and prevents excessive voltage fromdamaging the electrical components (further illustrated with regards toFIG. 4) comprising the standby power supply 112.

It should be appreciated that in some examples of the present invention,the clamp reset circuit 106 may be found within the standby power supply112 rather than the main power supply 104. However, the main powersupply 112 may electrically couple to the clamp reset circuit 106. Inother examples, the clamp reset circuit 106 is a separate entity towhich both the main power supply 104 and the standby power supply 112electrically couple. By allowing the main power converter 104 and thestandby power supply 112 to share the same clamp reset circuit 106, theteachings of the present invention may utilize fewer components and mayprovide a lower cost system than standard main and standby power supplydesigns used in electrical equipment, such as PC power supplies,printers, or the like.

Referring next to FIG. 2, a schematic diagram is illustrated of a mainpower supply 104 utilized with the power supply system including clampreset circuit 106, input (V_(IN)) 108, main output (V_(MO)) 110, standbypower supply 112, clamp standby connection 116, input return 118, atransformer T1 202, a primary winding 204 of transformer T1 202, asecondary winding 206 of transformer T1 202, two active switches S1 208and S2 210, two passive switches D1 212 and D2 214, an output diode D3216, a freewheeling diode D4 218, an output inductor L1 220, an outputcapacitor C1 222, and a load 224.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, and input return118 are electrically coupled as discussed with respect to FIG. 1. Asmentioned above, the main power supply 104 may be designed with atwo-switch forward converter topology. The two-switch forward converteris often the lowest cost configuration suited for power supplies forpersonal computers and similar applications. However, it should beappreciated that other topologies for the main power supply 104 maybenefit from the teachings of the present invention.

An active switch normally receives a control signal which switchesbetween states to control the opening and closing of the active switch,whereas a passive switch does not receive a control signal to switchbetween an open and closed state. An open switch normally does notconduct current. A closed switch can conduct current. Active switchestypically have one or more control terminals that determine whether ornot two other terminals of the active switch may conduct current. Thecontrol signals that open and close active switches S1 208 and S2 210(as illustrated in FIG. 2 and in subsequent figures depicting activeswitches) are not shown to help avoid obscuring more important detailsof the invention.

Passive switches generally have only two terminals. Typically, thevoltage between the terminals determines whether a passive switch isopen or closed. A diode is sometimes considered a passive switch,because it generally conducts current when the voltage between its twoterminals has one polarity (anode positive with respect to cathode), andsubstantially blocks current when the voltage between the terminals hasthe opposite polarity (anode negative with respect to cathode).

The main power supply 104 further comprises transformer T1 202, primarywinding 204 of transformer T1 202, secondary winding 206 of transformerT1 202, two active switches S1 208 and S2 210, two passive switches D1212 and D2 214, an output diode D3 216, a freewheeling diode D4 218,output inductor L1 220, and output capacitor C1 222 coupled together ina two-switch topology with reset capabilities illustrated herein. Asshown, the active switch S1 208 is coupled to input (V_(IN)) 108 andpassive switch D1 212 is also coupled to input (V_(IN)) 108 through theclamp reset circuit 106. The active switch S2 210 and the passive switchD2 214 are both coupled to the input return 118. However, it should beappreciated that variants of the example power converter topology may beutilized with the teachings of the present invention. Clamp resetcircuit 106 is coupled to passive switch D1 212 and to input (V_(IN))108. However, it should be appreciated that in some embodiments theclamp reset circuit 106 may be coupled to input return 118 asillustrated with regards to FIG. 3B. The output is provided from themain power supply 104 through the main output (V_(MO)) 110, exemplifiedas the voltage across the output capacitor C1 222, and provided to aload 224 external from the power supply 102. The load 224 includes otherelectrical components receiving power from the power supply system 100,such as motherboards or hard disk drives. Although in some embodiments,the load 224 may be internal to the power supply 102.

The main power supply 104 utilizing the two-switch forward convertertopology comprises two active switches, S1 208 and S2 210, with twopassive switches, D1 212 and D2 214, and clamp reset circuit 106 in aconfiguration that produces a voltage on a primary winding 204 of atransformer T1 202 from an input (V_(IN)) 108. A secondary winding 206of the transformer T1 202 produces a voltage proportional to the voltageon a primary winding 204 of a transformer T1 202. An output diode 216rectifies the voltage at the secondary winding 206. A freewheeling diodeD4 218 establishes a path for current in the output inductor L1 when theoutput diode D3 216 is reverse biased. An output inductor L1 220, and anoutput capacitor C1 222 filter the rectified voltage from the secondarywinding 206 to produce a main output (V_(MO)) 110 at the load 224. Asshown, primary winding 204 is galvanically isolated from secondarywinding 206. In particular, galvanic isolation prevents DC current fromthe primary side circuitry (circuitry electrically coupled to theprimary winding 204) from being received by secondary circuitry(circuitry electrically coupled to the secondary winding 206). However,it should be appreciated that the primary winding 204 need not begalvanically isolated from the secondary winding 206.

As illustrated, the main power supply 104 utilizes the clamp resetcircuit 106 along with the two-switch forward converter topology. Theclamp reset circuit 106 manages the magnetizing energy and leakageinductance of the transformer T1 202 of the main power supply 104. Thetwo-switch forward converter configuration allows the magnetic energy ofthe transformer T1 202 to reset (that is, return to a much lower value)when the active switches S1 208 and S2 210 are off. Resetting (or inother words reducing) the magnetizing energy of the transformer T1 202prevents excess stored energy from saturating the core material of thetransformer T1 202 and thereby altering its properties. The reset isgenerally achieved by applying a reset voltage of appropriate magnitudeand duration to the primary winding 204 when the active switches S1 208and S2 210 are off. It is often desirable to set the reset voltage to ahigher value than the voltage which appears on the primary winding 204when the active switches S1 208 and S2 210 are on. Setting the resetvoltage to a higher value than the voltage which appears on the primarywinding 204 when the active switches S1 208 and S2 210 are on allows forfaster reset. The clamp reset circuit 106 develops a substantiallyconstant voltage which is applied to the primary winding during thereset time of the transformer T1 202. In the example of the two-switchforward converter illustrated in FIG. 2, the reset voltage is a sum ofthe input voltage (V_(IN)) 108 and the voltage on the clamp resetcircuit 106. For the main power supply 104, the clamp reset circuit 106increases the voltage on the primary winding 204 of the transformer T1202 when the passive switches, D1 212 and D2 214, are conducting currentand the active switches S1 208 and S2 210 are open. As mentioned above,the clamp reset circuit 106 is also electrically coupled to the standbypower supply 112 through clamp standby connection 116 and is utilized tomanage the leakage inductance energy and to prevent excessive voltagefrom damaging the components comprising the standby power supply 112. Inother words, the clamp reset circuit 106 “clamps” the voltage forcomponents of the standby power supply 112 within acceptable tolerancelevels.

Generally, the main power supply 104 and the standby power supply 112within a power supply are two separate power supplies sharing the sameinput and providing their respective outputs. The main power supply 104and the standby power supply 112 share the clamp reset circuit 106, inaccordance with the teachings of the present invention, and theconfiguration of the main power supply 104 and the standby power supply112 discussed may lower solution cost.

Referring to FIG. 3A, another schematic diagram of a main power supply104 utilized with the power supply system 100 is illustrated includingclamp reset circuit 106, input (V_(IN)) 108, main output (V_(MO)) 110,standby power supply 112, clamp standby connection 116, input return118, a transformer T2 302, a primary winding 304 of transformer T2 302,a secondary winding 306 of transformer T2 302, an active switch S3 308,a passive switch D5 310, an output diode D6 312, a freewheeling diode D7314, an output inductor L2 316, an output capacitor C2 318, and a load320.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, and input return118 are electrically coupled and function as discussed above withrespect to FIG. 1. Unlike the main power supply 104 illustrated in FIG.2, the main power supply 104 illustrated in FIG. 3A utilizes asingle-switch forward converter topology. The main power supplycomprises transformer T2 302, primary winding 304 of transformer T2 302,secondary winding 306 of transformer T2 302, active switch S3 308,passive switch D5 310, an output diode D6 312, a freewheeling diode D7314, output inductor L2 316, output capacitor C2 318, and load 320coupled together in a single-switch topology with reset capabilitiesillustrated herein. As shown, primary winding 304 is galvanicallyisolated from secondary winding 306. However, it should be appreciatedthat the primary winding 304 need not be galvanically isolated from thesecondary winding 306. Clamp reset circuit 106 is coupled to the cathodeof the passive switch D5 310 and the input (V_(IN)) 108. However, itshould be appreciated that the clamp reset circuit 106 may alternativelybe coupled to input return 118 as shown with respect to FIG. 3B. Theoutput is provided from the main power supply 104 through the mainoutput (V_(MO)) 110, exemplified as the voltage across the outputcapacitor C2 318, and provided to the load 320 external from the powersupply 102. Although in some embodiments, the load 320 may be internalto the power supply 102.

The main power supply 104 utilizing the single-switch forward convertertopology comprises active switch S3 308 with passive switch D5 310 andclamp reset circuit 106 in a configuration that produces a voltage on aprimary winding 304 of a transformer T2 302 from an input (V_(IN)) 108.A secondary winding 306 of the transformer T2 302 produces a voltageproportional to the voltage on a primary winding 304 of a transformer T2302. Components coupled to the secondary winding 306 of the transformerT2 302 function similarly to the components coupled to the secondarywinding 206 of transformer T1 202 as discussed with respect to FIG. 2.The output diode D6 312, freewheeling diode D7 314, output inductor L2316, and output capacitor C2 318 provide substantially similarfunctionality as the output diode D3, freewheeling diode D4, outputinductor L1 220 and output capacitor C1 222 of the main power supply 104utilizing the two-switch forward converter topology discussed above.

The clamp reset circuit 106 manages the magnetizing energy and leakageinductance of the transformer T2 302 of the main power supply 104. Thesingle-switch forward configuration allows the magnetizing energy of thetransformer T2 to reset, or in other words reduce the magnetizingenergy, when the active switch S3 308 is off and prevents excess storedenergy from saturating the core material of the transformer T2. Asmentioned above, the reset is generally achieved by applying a resetvoltage to the primary winding 304 when the active switch S3 308 is offand it is often desirable to set the reset voltage to a higher valuethan the voltage which appears on the primary winding 304 when theactive switch S3 is on. The clamp reset circuit 106 develops asubstantially constant voltage which is applied to the primary windingduring the reset time of the transformer T2. For the example of thesingle-switch forward converter illustrated in FIG. 3A, the resetvoltage is the voltage on the clamp reset circuit 106. For the mainpower supply 104 utilizing the single-switch forward converter topology,the clamp reset circuit 106 sets the voltage on the primary winding 304of the transformer T2 302 when the passive switch D5 310 is conductingcurrent.

FIG. 3B illustrates another schematic diagram of a main power supply 104utilized with the power supply system 100 is illustrated including clampreset circuit 106, input (V_(IN)) 108, main output (V_(MO)) 110, standbypower supply 112, clamp standby connection 116, input return 118, atransformer T2 302, a primary winding 304 of transformer T2 302, asecondary winding 306 of transformer T2 302, an active switch S3 308, apassive switch D5 310, an output diode D6 312, a freewheeling diode D7314, an output inductor L2 316, an output capacitor C2 318, and a load320.

Similar to the main power supply 104 discussed with respect to FIG. 3A,the main power supply 104 utilizes a single-switch forward convertertopology. The components of FIG. 3B function substantially similar tothe components introduced in FIG. 3A, however, the clamp reset circuit106 is alternatively coupled to the cathode of passive switch D5 310 andinput return 118. In other words the clamp reset circuit 106 may becoupled to the cathode of passive switch D5 310 and the negativeterminal of the input (V_(IN)) 108.

As mentioned above, the clamp reset circuit 106 manages the magnetizingenergy and leakage inductance of the transformer T2 302 of the mainpower supply 104 which allows the magnetizing energy of the transformerT2 302 to reset. For the example of the single-switch forward converterillustrated in FIG. 3B, the reset voltage is the difference of thevoltage on the clamp reset circuit 106 and input voltage (V_(IN)) 108.

FIG. 4 illustrates a schematic diagram of a standby power supply 112utilized with the power supply system 100 comprising main power supply104, clamp reset circuit 106, input (V_(IN)) 108, standby power supply112, standby output (V_(SO)) 114, clamp standby connection 116, inputreturn 118, a coupled inductor T3 402, a primary winding 404 of thecoupled inductor T3 402, a secondary winding 406 of the coupled inductorT3 406, an active switch S4 408, a passive switch D8 410, a rectifier D9412, an output capacitor C3, and a load 416. However, it should beappreciated that other topologies for the standby power supply 112 maybenefit from the teachings of the present invention.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, and input return118 are electrically coupled as discussed with respect to FIG. 1. Asmentioned above, the standby power supply 112 is typically designed witha flyback converter topology utilizing a clamp circuit. However, itshould be appreciated that a forward converter topology (such as thetwo-switch or single switch forward converter illustrated above), atwo-switch flyback converter topology, or other converter topology couldalso be utilized to design the standby power supply 112. The flybackconverter topology often provides a low cost solution suited for astandby power supply.

The standby power supply 112 further comprises coupled inductor T3 402,primary windings 404 of the coupled inductor T3 402, secondary windings406 of the coupled inductor T3 406, active switch S4 408, passive switchD8 410, rectifier D9 412, output capacitor C3, and load 416 coupledtogether in a flyback converter topology with clamp capabilitiesillustrated herein. Clamp reset circuit 106 is coupled to the cathode ofthe passive switch D8 410. Further, the clamp reset circuit 106 mayfurther be coupled to the positive terminal of input (V_(IN)) 108. Inother embodiments, the clamp reset circuit 106 is further coupled toinput return 118. Or in other words, the clamp reset circuit 106 mayfurther be coupled to the positive or negative terminal of input(V_(IN)) 108. The clamp reset circuit 106 couples to the standby powersupply 112 through clamp standby connection 116. The output is providedfrom the standby power converter 112 through the standby output (V_(SO))114, exemplified as the voltage across the output capacitor C3 414, andprovided to a load 416 external from the power supply 102. Although insome embodiments, the load 416 may be internal to the power supply 102.

The standby power supply 112 utilizing the flyback converter topologycomprises active switch S4 408 with passive switch D8 410 and clampreset circuit 106 in a configuration which produces a voltage on theprimary winding 404 of the coupled inductor T3 402 from an input(V_(IN)) 108. A secondary winding 406 of the coupled inductor T3 402produces a voltage proportional to the voltage on the primary winding404 of the coupled inductor T3 402. The output diode D9 412 rectifiesthe voltage at the secondary winding 406, and an output capacitor C3filters the current from the output diode D9 412 to produce the standbyoutput (V_(SO)) 114 at the load 416. As shown, primary winding 404 isgalvanically isolated from secondary winding 406. However, it should beappreciated that the primary winding 404 need not be galvanicallyisolated from the secondary winding 406. As mentioned above, the standbypower supply 112 is further coupled to the clamp reset circuit 106through the clamp standby connection 116. The clamp reset circuit 106allows the standby supply to manage the leakage inductance energy withinthe coupled inductor T3 and limits the maximum voltage on the activeswitch S4 408 when the passive switch D8 is conducting current. The mainpower supply 104 and the standby power supply 112 share the clamp resetcircuit 106, in accordance with the teachings of the present invention,and the configuration discussed may provide lower cost benefits.

FIG. 5A illustrates one example of a schematic diagram of the clampreset circuit 106 utilized with the power supply system 100 comprisingmain power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, input return118, a Zener diode 502, a capacitor 504, and a resistor 506.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, and input return118 are electrically coupled as discussed with respect to the previousfigures. The Zener diode 502 is electrically coupled to the resistor506; the Zener diode 502 and the resistor 506 are then electricallycoupled to the capacitor 504. The clamp reset circuit 106 is coupled tothe standby power supply 112 through the clamp standby connection 116.The standby power supply 112 electrically couples to the clamp resetcircuit 106 at a point between the passive switch D1 212, when (forexample) the configuration illustrated in FIG. 2 is utilized, and theclamp reset circuit 106. In another embodiment, the standby power supply112 electrically couples to the clamp reset circuit 106 at passiveswitch D5 as illustrated in FIG. 3A. In other words, the standby powersupply 112 electrically couples to the clamp reset circuit 106 at apoint between the clamp reset circuit 106 and the input return 118.However, it should be appreciated that the main power supply 104 and thestandby power supply 112 can be coupled to the clamp reset circuit usingany number of configurations depending on the circuit topology utilizedfor the main and standby power supply 104 and 112.

As mentioned above, the clamp reset circuit 106 manages the magnetizingenergy and leakage inductance of the transformer within the main powersupply 104. Both the single switch configuration and the two switchconfiguration allow the magnetizing energy of the transformer to reset(in other words, return to a much lower value) when the active switchesare off. Resetting the magnetizing energy of the transformer preventsexcess stored energy from saturating the transformer. The reset isgenerally achieved by applying a reset voltage of appropriate magnitudeand duration to the primary winding when the active switches are off.When a two-switch forward converter is used, the clamp reset circuit 106allows the voltage on the primary winding of the main power supply 104to increase when the passive switches are conducting current. On theother hand, the clamp reset circuit 106 prevents excess voltage fromdamaging components to occur in the standby power supply 112. In otherwords, the clamp reset circuit 106 limits the voltage across the activeswitch of the standby power supply 112 with a capacitor 504 whichmaintains a substantially constant voltage limited by the Zener diode502. In general, the Zener diode 502 limits the voltage across thecapacitor 504. The Zener diode 502 limits the energy received by thecapacitor 504 from exceeding a threshold. The threshold typicallycorresponds to properties of the Zener diode 502, such as the Zenerbreakdown voltage.

In choosing the properties of the Zener diode 502, capacitor 504 andresistor 506, both the requirements of the main power supply 104 and thestandby power supply 112 should be taken into account. Unlike the mainpower supply 104, the standby power supply 112 is constantly operating.As a result, the capacitor 504 within the clamp reset is constantlyrefreshed with energy from the leakage inductance of the standby powersupply transformer (or coupled inductor). The capacitor 504 may remaincharged regardless of whether the main power supply 104 is operating.With the capacitor 504 charged, the main power supply 104 can bring themain output (V_(MO)) 110 into regulation faster than if the capacitor504 was not charged.

By sharing the clamp reset circuit 106, the standby power supply 112generates a stable clamp voltage. In general, the main power supply 104eventually shuts off (usually in response to a user indication) whilethe standby power supply 112 remains constantly operating. Since thestandby power supply 112 is constantly running, the standby power supply112 provides a stable clamp voltage in the clamp reset circuit 106. Themain power supply 104 advantageously uses the stable clamp voltage forresetting the energy within the transformer of the main power supply104. As a result, the main power supply 104 may even operate underno-load conditions. By sharing the components of the clamp reset circuit106, the main power supply 104 may rely on the stable clamp voltageregardless of the load conditions on the main power supply 104.

FIG. 5B illustrates another schematic diagram of a clamp reset circuit106 utilized with the power supply system 100 including main powersupply 104, clamp reset circuit 106, input (V_(IN)) 108, standby powersupply 112, clamp standby connection 116, input return 118, a Zenerdiode 502, a capacitor 504, and a clamp reset resistor 506, and astandby current limiting resistor 508.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,standby power supply 112, clamp standby connection 116, and input return118 are electrically coupled as discussed with respect to the previousfigures. Components of the clamp reset circuit 106, such as the Zenerdiode 502, capacitor 504, and clamp reset resistor 506 function and arecoupled substantially as described with respect to FIG. 5A.Additionally, standby current limiting resistor 508 is coupled betweenthe passive switch D8 212 and the clamp reset circuit 106.

Standby current limiting resistor 508 may be coupled to passive switchD8 when a slow diode is utilized for the passive switch. Passiveswitches may comprise PN junction diodes. When the polarity of thevoltage on a PN junction diode transitions to change the state of thediode from conducting current to blocking current (otherwise known as anON or OFF state), there is a momentary reverse current for a period oftime known as the reverse recovery time. The length of the reverserecovery time characterizes whether a diode is a slow or a fast diode. Adiode which has a short reverse recovery time is considered to be a fastdiode. A diode having a long reverse recovery time can be known as aslow diode. A fast diode typically has a reverse recovery timesubstantially less than one microsecond. A slow diode typically has areverse recovery time greater than one microsecond. In some cases, alarge amount of uncontrolled reverse current may dissipate power andlower the efficiency of the power supply. The use of a current limitingresistor (such as resistor 508) with a slow diode limits the amount ofreverse current passing through passive switch D8 410. Although notillustrated, an additional current limiting resistor may be utilized inseries with the passive switch D1 212 when the passive switch D1 212 isa slow diode. It should be appreciated that the power supply system 100may utilize either the standby current limiting resistor 508, or theadditional current limiting resistor coupled to passive switch D1 212,or both when slow diodes are utilized for either passive switches D8 410or D1 212, or both.

FIG. 5C illustrates a further schematic diagram of a clamp reset circuit106 utilized with the power supply system 100 including main powersupply 104, clamp reset circuit 106, input (V_(IN)) 108, standby powersupply 112, clamp standby connection 116, input return 118, a Zenerdiode 502, a capacitor 504, and a resistor 506.

FIG. 5C illustrates a clamp reset circuit 106 coupled to the inputreturn 118 as discussed with respect to FIG. 3B. The main power supply104, clamp reset circuit 106, input (V_(IN)) 108, standby power supply112, clamp standby connection 116, and input return 118 are electricallycoupled as discussed with respect to the previous figures. The Zenerdiode 502 is electrically coupled to the resistor 506; the Zener diode502 and the resistor 506 are then electrically coupled to the capacitor504. The standby power supply 112 electrically couples to the clampreset circuit 106 at a point between the passive switch D5 310, when(for example) the configuration illustrated in FIG. 3B is utilized andcapacitor 504. In other words, the standby power supply 112 electricallycouples to the clamp reset circuit 106 at a point between the clampreset circuit 106 and the input return 118. The clamp reset circuit 106further couples to the input return 118 or the negative terminal of theinput (V_(IN)) 108. In addition, a current limiting resistor, asillustrated above with respect to FIG. 5B, may also be utilized with theclamp reset circuit 106 of FIG. 5C when slow diodes are utilized for anyof the passive switches, such as passive switch D5 310.

FIG. 6 illustrates another schematic diagram of a main power supply 104including clamp reset circuit 106, input (V_(IN)) 108, main output(V_(MO)) 110, standby power supply 112, clamp standby connection 116,input return 118, a transformer T1 202, a primary winding 204 oftransformer T1 202, a secondary winding 206 of transformer T1 202, twoactive switches S1 208 and S2 210, two passive switches D1 212 and D2214, output diode D3 216, a freewheeling diode D4 218, an outputinductor L1 220, output capacitor C1 222, load 224, diode D10 602,resistance 604, and capacitor 606.

The main power supply 104, clamp reset circuit 106, input (V_(IN)) 108,main output (V_(MO)) 110, standby power supply 112, clamp standbyconnection 116, and input return 118 couple and function as discussedabove with respect to FIG. 1. In addition, the transformer T1 202,primary winding 204 of transformer T1 202, secondary winding 206 oftransformer T1 202, two active switches S1 208 and S2 210, two passiveswitches D1 212 and D2 214, output diode D3 216, freewheeling diode D4218, output inductor L1 220, output capacitor C1 222, and the load 224couple and function as discussed with respect to FIG. 2. As illustratedin FIG. 6, the main power supply 104 is designed with the two-switchforward converter topology (as illustrated in FIG. 2) with the addedcomponents of diode D10 602, resistance 604, and capacitance 606.

The diode D10 602 is coupled to the primary winding 204 of transformerT1 202 and passive switch D1 212. The resistance 604 is coupled betweendiode D10 602 and passive switch D1 212. When a diode is utilized forpassive switch D1 212, resistance 604 is coupled the cathode end of bothdiode D10 602 and passive switch D1 212. One end of capacitance 606 iscoupled to the diode D10 602 and resistance 604 while the other end ofcapacitance 606 is coupled to input (V_(IN)) 108. In other words, theclamp reset circuit 106 is coupled across capacitance 606 and resistance604.

In some embodiments of the present invention, diode D10 602, resistance604, and capacitance 606 may be added to the main power supply to limitvoltage spikes due to leakage inductance energy and magnetizing energyof the transformer T1 202. FIG. 6 illustrates the main power supply 104illustrated in the two-switch forward converter topology. However, itshould be appreciated that the added diode D10 602, resistance 604, andcapacitance 606 may be utilized with various other topologies of themain power supply 104. As mentioned above, a diode may be used forpassive switch D1 212. Depending on the length of the reverse recoverytime of the diode, the diode may be considered a slow diode or a fastdiode. In some embodiments, passive switch D1 212 is a slow diode asdiscussed with respect to FIG. 5B. Further, in some embodiments passiveswitch D2 214 is also a slow diode. It should be appreciated that acurrent limiting resistor (not shown) is coupled to passive switch D1212 and passive switch D2 214 when either passive switch D1 212 orpassive switch D2 214 are slow diodes as discussed with FIG. 5B. In thetwo-switch forward converter topology (without added diode D10 602,resistance 604, and capacitance 606) a voltage spike occurs from theleakage inductance energy and the magnetizing energy of transformer T1202 due to the length of the forward recovery time of passive switch D1212. In some embodiments, the voltage spike is within the acceptablelimits of operation. However, when the voltage spike is not within theacceptable limits of operation, the diode D10 602, resistance 604, andcapacitance 606 may be included to limit the voltage spike.

When the passive switch D1 212 is a diode, in operation it takes acertain amount of time for the passive switch D1 212 to switch from areverse biased diode to a forward biased diode. This amount of time istypically referred to as the forward recovery time of the diode. Duringthe forward recovery time of passive switch D1 212, the clamp resetcircuit 106 is not electrically connected to the passive switch D1 212and a voltage spike occurs from the leakage inductance energy oftransformer T1 202. Once the forward recovery time of passive switch D1212 has passed and passive switch D1 212 is forward biased, the clampreset circuit 106 is electrically connected to the passive switch D1 212and may manage the magnetizing energy and the leakage inductance energyof the transformer T1 202. By adding the diode D10 602, resistance 604and capacitance 606, the voltage spike may be limited during the forwardrecovery time of diode D1 212.

In operation, when diode D10 602 is forward biased, the leakageinductance energy and the magnetizing energy of transformer T1 202 maybe stored within capacitance 606. In one embodiment, diode D10 602 is afast diode which switches from reverse bias to forward biassignificantly quicker than passive switch D1 212. In some embodiments,diode D10 602 is an ultra fast diode. Since diode D10 602 has asignificantly shorter forward recovery time than passive switch D1 212,the leakage inductance energy and magnetizing energy of transformer T1202 may be stored by capacitance 606 to limit the voltage spike. Oncepassive switch D1 212 is forward biased, the clamp reset circuit 106 maymanage the magnetizing energy and the leakage inductance energy oftransformer T1 202.

Resistance 604 allows the main power supply 104 to recover the energystored by capacitance 606. When passive switches D1 212 and D2 214 alongwith diode D10 602 switch from forward bias to reverse bias, resistance604 provides a path for current from capacitance 606 through passiveswitch D1 212, transformer T1 202 and passive switch D2 214. In oneembodiment, diode D10 602 is an ultra fast diode with a reverse recoverytime significantly shorter than the reverse recovery time of passiveswitch D1 212. As such, diode D10 602 is reverse biased quicker thanpassive switch D1 212 allowing current from capacitance 606 to flowthrough resistance 604 and passive switch D1 212. As current fromcapacitance 606 flows through transformer T1 202, energy stored by thecapacitance 606 is recovered by the main power supply 104. In addition,energy stored by the clamp reset circuit 106 may also be recovered bythe main power supply 104 as current from the clamp reset circuit 106flows through passive switch D1 212. By adding the diode D10 602,resistance 604 and capacitance 606, voltage spikes due to leakageinductance energy and magnetizing energy may be reduced.

While the invention herein disclosed has been described by means ofspecific embodiments, examples and applications thereof, numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the scope of the invention set forth inthe claims.

What is claimed is:
 1. A power supply comprising: a first powerconverter adapted to convert an input to a first voltage output, thefirst power converter including a first transformer having a firstprimary winding; a second power converter adapted to convert the inputto a second voltage output, the second power converter including asecond transformer having a second primary winding that is not the firstprimary winding of the first transformer, wherein the first voltageoutput is separate from the second voltage output; and a shared clampreset circuit coupled to the first primary winding of the firsttransformer and coupled to the second primary winding of the secondtransformer, wherein the clamp reset circuit is adapted to manageleakage inductance energy within the first transformer and within thesecond transformer wherein the clamp reset circuit further comprises: acapacitor, wherein the capacitor is adapted to store energy receivedfrom the first power converter and the second power converter; and aZener diode coupled to the capacitor, wherein the Zener diode is adaptedto prevent the energy received from the first power converter and thesecond power converter to exceed a threshold and the Zener diode limitsvoltage on the capacitor.
 2. The power supply of claim 1, wherein theclamp reset circuit further limits voltage on a component of the secondpower converter.
 3. The power supply of claim 1, wherein the secondpower converter is arranged to remain powered up while the first powerconverter is powered down.
 4. The power supply of claim 1, wherein thefirst power converter comprises a passive switch coupled to the clampreset circuit.
 5. The power supply of claim 4, wherein the passiveswitch is a slow diode.
 6. The power supply of claim 5, wherein thefirst power converter further comprises a resistor coupled to the clampreset circuit and the slow diode, wherein the resistor is adapted tomanage current through the slow diode.
 7. The power supply of claim 1,wherein the first power converter further comprises circuitry in atwo-switch forward converter topology.
 8. The power supply of claim 1,wherein the first power converter further comprises circuitry in asingle-switch forward converter topology.
 9. The power supply of claim1, wherein the second power converter comprises a passive switch coupledto the clamp reset circuit.
 10. The power supply of claim 9, wherein thepassive switch is a slow diode.
 11. The power supply of claim 10,wherein the second power converter further comprises a resistor coupledto the clamp reset circuit and the slow diode, wherein the resistor isadapted to manage current through the slow diode.
 12. The power supplyof claim 1, wherein the second power converter further comprisescircuitry in a flyback converter topology.
 13. The power supply of claim1, wherein the second power converter further comprises circuitry in atwo-switch forward converter topology.
 14. The power supply of claim 1,wherein the second power converter further comprises circuitry in asingle-switch forward converter topology.
 15. The power supply of claim1, wherein the second power converter further comprises circuitry in atwo-switch flyback converter topology.
 16. The power supply of claim 1,wherein components of the clamp reset circuit are included in the firstpower converter.
 17. The power supply of claim 1, wherein components ofthe clamp reset circuit are included in the second power converter. 18.The power supply of claim 1, wherein the clamp reset circuit is coupledto a first terminal of the input which has a positive polarity withrespect to a second terminal of the input.
 19. The power supply of claim1, wherein the clamp reset circuit is coupled to a second terminal ofthe input which has a negative polarity with respect to a first terminalof the input.